Effect of Cobalt Ion Concentration and Thermal Annealing Temperature on Structural and Magnetic Properties of CoFe₂O₄ Nanoparticles

Exploring the physical properties of magnetic nanoferrites for applications in data storage media and biomedicine is a crucial step, providing new insights into the physics of nanostructured materials. Here, the focus is on studying the effect of cobalt ion concentration and thermal annealing temperature on the structural and magnetic properties of cobalt ferrite (CoFe₂O₄) nanoparticles (NPs) synthesized using a co-precipitation method. To this end, Co_1−x(Fe₂O₄)_x (x = 0.25, 0.5, and 0.75 M) NPs are initially prepared and then thermally annealed at different temperatures (T = 400–800 °C). X-ray diffraction patterns along with field-emission scanning electron microscopic images indicate the formation of inverse cubic spinel structure with different crystallite sizes and NP size distributions when changing the cobalt ion concentration. Based on hysteresis loop measurements, magnetic parameters such as saturation magnetization (M_s) and coercivity (H_c) show increasing trends from 5.641 emu/g and 146.246 Oe to 8.936 emu/g and 1789.555 Oe when decreasing the cobalt ion concentration. By performing the annealing process, magnetic properties are significantly enhanced in the case of x = 0.25 and 0.5 at T = 400 °C and 600 °C, achieving M_s = 129.954 emu/g and H_c = 1137.697 Oe. Meanwhile, first-order reversal curve (FORC) diagrams are employed to map magnetostatic interactions and coercivity distributions as a function of cobalt ion concentration for NPs annealed at T = 400 °C, manifesting magnetically soft and hard phases. It is found the maximum FORC distribution shifts to higher H_c values with decreasing cobalt ion concentration.